Hitherto, various accessory machines mounted on an engine have been driven by various methods. When a gear driving system is used, timing gears for transmitting the driving torque of the crank shaft to the cam shaft, a fuel injection pump, and the like, are disposed and housed in a timing gear case of the engine in a state in which the timing gears are meshed with each other. Most construction machines, and large vehicles, such as buses or trucks, have a diesel engine mounted therein which generates a large torque and whose fuel cost is economical. The diesel engine for these vehicles is provided with a driven engine accessory, e.g., a driven fuel injection pump. In one type, a fuel injection pump is mounted in a cylinder block of an engine or on the side of a crank case, and the fuel injection pump is driven by a fuel injection pump driving gear located inside a timing gear case. In another type of driven fuel injection pump, the fuel injection pump is directly mounted on the external surface of a timing gear case, and is driven by the fuel injection pump driving gear. From among types in which the former fuel injection pump is mounted in the cylinder block or on the side of the crank case, as means for restraining variations in a driving torque, there is a type in which a weight having a large rotational inertia is disposed in the middle of the driving shaft. A damper formed of an inertia ring member and an elastic member is added to a coupling of the fuel injection pump driving shaft and a fuel injection pump shaft, thereby reducing torsion vibrations which are generated in the driving shaft system (refer to, for example, Japanese Utility Model Laid-Open No. 1-180021).
However, in order to cope with recent exhaust emission regulations, there is a tendency for the fuel injection pressure of the diesel engine to be higher. Therefore, since the driven torque of the fuel injection pump increases, problems are assumed to arise, for example, the gear grating noise between the driving gears which mesh with each other increases, the tooth surfaces are damaged, or the driving shaft system may malfunction, for example, the coupling and the driving shaft may be broken. FIGS. 8a to 8h show an example for comparing an engine having mounted thereon a fuel injection pump, which is set at a normal fuel injection pressure, with an engine having a fuel injection pump whose fuel injection pressure is set at a high pressure, regarding changes in various characteristics as the fuel injection pressure becomes higher. As a result of the fuel injection pump becoming higher in pressure, the pressure of the fuel injection pipe increases (see FIGS. 8a and 8b). The variation waveform of the driving torque of the fuel injection pump becomes steep in proportion to the pressure waveform of the fuel injection pipe, and its absolute value increases (see FIGS. 8c and 8d). In response to this increase, the rotation variation of the driving gear of the fuel injection pump becomes greater (see FIGS. 8e and 8f). Further, the impact force between the tooth surfaces between the driving gear of the fuel injection pump and the idle gear, which meshes with the driving gear, increases and the surface vibrations of the gear case and the surface vibrations of the cylinder block increase (see FIGS. 8g and 8h). Also, the increase in the variation in the driving torque can cause an excessive stress to occur in the driving shaft, causing problems, for example, the driving system may be damaged or broken. Therefore, in some cases, it is necessary to relax the driven torque which varies sharply, including the driving torque.